Oxy-combustion is being developed for carbon dioxide capture and sequestration in fossil fuel fired power plants. The concept of oxy-combustion (also sometimes referred to as ‘oxyfuel’ and ‘oxy-firing’) is to replace combustion air with a mixture of oxygen and recycled flue gas, thereby creating a high carbon dioxide content flue gas stream that can be more simply processed for sequestration. A simplified exemplary schematic of the oxy-combustion process for pulverized coal (pc) power plants is shown in prior art depicted in FIG. 1.
FIG. 1 depicts an oxy-combustion system 100, comprising an air separation unit 102, a boiler 104, a pollution control system 106 and a gas processing unit 108. The air separation unit 102 is located upstream of the boiler 104, which is located upstream of the pollution control system 106 and the gas processing unit 108. The pollution control system 106 is located upstream of the gas processing unit 108. Gas recycle is shown taken after the pollution control system, but could be taken from any location between the boiler and the gas processing unit.
The boiler 104 may be a tangentially fired boiler (also known as a T-fired) or a wall fired boiler. T-firing is different from wall firing in that it utilizes burner assemblies with fuel admission compartments located at the corners of the boiler furnace, which generate a rotating fireball that fills most of the furnace cross section. Wall firing (not shown), on the other hand, utilizes burner assemblies that are perpendicular to a side (of the shell) of the boiler.
FIG. 2 depicts a tangentially fired boiler 104. Tangentially fired boilers have a rectangular cross-section and have burner assemblies 105 positioned at the corners. Fuel and transport air are introduced into the boiler 104 via the burner assemblies 105 and are directed tangentially to an imaginary circle located at the center of the furnace and with a diameter greater than zero. This generates a rotating fireball that fills most of the furnace cross section. The fuel and air mixing is limited until the streams join together in the furnace volume and generate a rotation. This has often been described, as “the entire boiler is the burner.” Global boiler aerodynamics and mixing is much more important to the combustion process and the resulting boiler performance during T-firing as compared with wall-firing. During wall-firing, fuel and air/oxygen mixing occurs in or near the burners and less mixing occurs in the boiler.
With reference now once again to FIG. 1, in one method of operating the oxy-combustion system 100, oxygen is first separated from nitrogen in the air separation unit 102. The nitrogen is discharged separately from the air separation unit. The air separation unit 102 extracts oxygen from the atmosphere.
The oxygen is then discharged from the air separation unit 102 to combine with recycled flue gas, the combination of which is fed to the boiler 104. The boiler 104 uses the oxygen present in the flue gas stream to combust a fuel (e.g., coal, oil, or the like) to generate heat and flue gases. As a result of combusting the fuel with oxygen instead of with air, the flue gas produced has a high carbon dioxide content. The other constituents of the flue gas are water vapor and small amounts of oxygen, nitrogen, and pollutants such as sulfur oxides, nitrogen oxides, and carbon monoxide. Removing the water and other components produces a very pure carbon dioxide stream suitable for sequestration or other use.
The heat is used to generate steam, which may be used to drive a generator (not shown) to produce electricity, while the flue gases are discharged to the pollution control system 106 where particulate matter and other pollutants (e.g., NOx, SOx, and the like) are removed. A portion of the purified flue gases is recycled to the boiler 104 as shown in FIG. 1. The remaining flue gases (that substantially comprises carbon dioxide) are discharged to the gas processing unit 108 from where it is sequestered.
If a large amount of recycled flue gases relative to the amount of oxygen is fed to the boiler 104 to effect combustion, the combustion temperatures reached in the boiler are not sufficient to facilitate the combustion of all of the fuel. In addition, larger equipment is needed to recycle such large amounts of flue gases. On the other hand, burning the fuel with pure oxygen generally produces flame temperatures much too high for practical boiler materials, so a portion of the high-carbon dioxide flue gas is used to dilute the oxygen and moderate the boiler temperature.
The amount of oxygen added to the recycled flue gas is based on the amount of fuel combusted in the boiler. The fuel uses a certain amount of oxygen in addition to some amount of excess oxygen to ensure complete combustion. The addition of the oxidant stream into the boiler at a single concentration in the boiler has certain disadvantages. One of these disadvantages is that the heat release or flux profile (hereinafter “heat release profile”) in the boiler is not optimized to produce the highest overall furnace heat absorption while maintaining acceptable conditions for tube metal temperatures, ash deposition and fireside corrosion that impact operating reliability and maintenance costs. It is therefore desirable to devise a methodology for introducing the oxidant stream into the boiler in such a manner so as to optimize the heat release profile in the boiler at different positions in the boiler so as to optimize the thermal performance of the boiler, to reduce ash deposition and fireside corrosion, and to prevent slagging and corrosion in the furnace.